This invention relates generally to thermal control blankets and, more particularly, to thermal control blankets used in spacecraft to reflect solar radiation, permit emission of radiation, and thereby prevent spacecraft payloads from being excessively heated. Blankets of this general type are often constructed by depositing on a substrate a coating that is reflective in the visible radiation spectrum. The substrate typically has a high emissivity in the mid- to far-infrared (IR) spectrum, because the bulk of the energy emitted from a spacecraft is from the infrared spectrum, at wavelengths greater than 1,000 nm.
U.S. Pat. No. 6,279,857 issued to James A. Roth and assigned to the same assignee as the present invention, describes the advantages of a silicon coating on a thermal control blanket. In brief, although silicon is not as good as germanium in reflecting energy in the visible spectrum, silicon is often preferred because it has better adhesion properties in relation to typical substrate materials, such as Kapton. As also mentioned in the Roth patent, germanium has the advantage of providing greater reflectivity than silicon, but it has inferior adhesion properties and is susceptible to corrosion on exposure to humidity.
Another issue of concern in the design of thermal control blankets is electrostatic discharge (ESD). Thermal control blankets for applications in space must be designed to have sufficient electrical conductivity to allow discharge of electrostatic charge that would otherwise build up and result in potentially damaging rapid discharge. Germanium films typically have a surface resistance of about 1 megohm/square, which provides reasonable protection from electrostatic charge build-up. On the other hand, silicon films typically have a sheet resistance on the order of 100 megohms/square, and need to be doped with an impurity to increase conductivity. Therefore, from an ESD perspective, germanium is preferred over silicon.
The Roth patent also suggests that the advantages of both materials can be obtained by applying them as multiple separate layers on the substrate. However, the use of multiple coatings on the substrate increases the complexity and cost of the manufacturing process, and poses potential problems arising from the presence of multiple interfaces between dissimilar materials. Ideally, a single thermal coating having the desired properties would be preferred, but neither silicon nor germanium provides the desired properties. Accordingly, there is still room for improvement over the thermal blanket structure disclosed and claimed in the Roth patent. The present invention is directed to this end.